Direct Measurement of the Femtosecond Reaction Dynamics of the CH4 · O3 van der Waals Complex


C. Cameron Miller, Roger D. van Zee and John C. Stephenson
National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Femtosecond time resolved studies have elucidated the mechanism for the reaction CH4 + O(1D) ®  CH3 + OH. Ozone in the CH4·O3 van der Waals cluster with photolyzed by 266 nm laser pulses, producing O(1D), and the appearance of OH (v=0,1; J) was determined by OH A ¬ X laser induced fluorescence as a function of pump/probe delay time. The appearance of OH A ¬ OH X was measured at six different probe wavelength regions ranging from 307.3 nm to 316.5 nm where each probe wavelength measures a different distribution of OH vibrational (v) and rotational (J) states. At each probe wavelength the reaction exhibits both a slow and fast formation time. The data supports a simple model in which OH is produced by two mechanisms: a slow mechanism where OH is formed from a CH3OH* intermediate in which the available energy randomizes and dissociates with a rate expected from RRKM theory; a fast mechanism where OH is formed from a CH3OH* intermediate which dissociates prior to energy randomization.